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LM311: No output detected

Genius 14669 points
Part Number: LM311
Other Parts Discussed in Thread: TL331

Hi Experts,

We'd like to ask assistance to troubleshoot this LM311 design from customer:

When thermistor bead is wet and compared to my set voltage pull low and allow output to connect to pin 4 and light up my LED light.
I am getting 0VDC at pin 7. Not understanding why.

Because my noninverting voltage (3.1V) is greater than my inverting voltage (2.1V), when the inverting voltage is lower than the noninverting voltage, the output should be drawn low. 

For your assistance. Thank you.

Archie A.

  • Hello Archie,

    It looks like you do not have the output and LED wired correctly.

    The LM311 has a unique output that can be used for either high-side or low-side loads (pull-up or pull-down). It looks like you are crossing the two, so it is not working.

    The left side is the more standard "common collector" output configuration, and the right is the less common "common emitter" output. The pullup/down resistor in the image would be the combination of the *series* LED and 2.2k dropping resistor. The comparator is designed that the transistor is conducting when the output is "low" (- > +).

    So when the bead is wet (+IN=3V, -IN=2.1V), the output will want to go HIGH (+IN>-IN), so the transistor is NOT conducting (off) and the LED is off.

    The opposite of what you seem to want - so simply reverse the +IN and -IN inputs.

    You seem to have it a little mixed up in the schematic.

    So do you wish the LED to be grounded? Then you need to use the "right" common emitter circuit and tie the collector to +28, and the emitter to the SERIES 2.2k to the LED, and then the LED to GND.

    You could also use the circuit on the left. This would make it more "universal" with other open collector comparators.

    Also, the LM311 input is not rail-to-rail, so at the lowest setting, pin 2 is pulled to GND and violates the minimum input voltage (0.5V). This may cause you problems later.

    Is there a particular reason you are using the LM311? This is not the device we would recommend for simple circuits. This is more suited for a TL331 type device .